Magnetic memory devices (HDDs) mainly used in computers to record and reproduce information have large capacities, inexpensiveness, high data access speeds, high data holding reliability, and the like, and hence are used in various fields such as household video decks, audio apparatuses, and automobile navigation systems. As the range of applications of the HDDs extends, demands for large storage capacities increase, and high-density HDDs are more and more extensively developed in recent years.
As a magnetic recording method of presently commercially available HDDs, a so-called perpendicular magnetic recording method is recently most frequently used. In the perpendicular magnetic recording method, magnetic crystal grains forming a magnetic recording layer for recording information have an axis of easy magnetization in a direction perpendicular to a substrate. The axis of easy magnetization is an axis in the direction of which magnetization easily points. In a Co-based alloy presently widely used as a magnetic recording layer material, the axis of easy magnetization is the axis (c-axis) parallel to the normal to the (0001) plane of the hcp structure of Co. In an ordered alloy having the L10 structure such as FePt, the axis of easy magnetization is the axis (c-axis) parallel to the normal to the (001) plane. As a recording layer of the existing perpendicular magnetic recording medium, a granular film type recording layer having a so-called granular structure in which magnetic crystal grains are surrounded by a grain boundary region made of a nonmagnetic material is widely used. The granular film type recording layer has a structure in which magnetic crystal grains are two-dimensionally, physically isolated by a nonmagnetic grain boundary region, so the magnetic exchange interaction acting between the magnetic grains reduces. In this recording layer, the lower limit of the recording bit size strongly depends on the magnetic crystal grain size of the granular film type recording layer. To increase the recording density, therefore, the magnetic crystal grain size must be decreased. If the exchange interaction between the magnetic crystal grains is very small, however, decreasing the magnetic crystal grain size deteriorates the thermal stability. On the other hand, to maintain the thermal stability of recording magnetization while decreasing the grain size of the magnetic crystal grains, a method of increasing the magnetic anisotropic energy (Ku) of the magnetic crystal grains can be used. However, the increase in Ku increases the magnetic anisotropic field (Hk). In the granular film type recording layer, the increase in Hk increases the coercive force (Hc). This increases a magnetic field required for magnetization reversal. That is, when increasing the recording density of the existing perpendicular magnetic recording medium, it is impossible to simultaneously solve the three problems, i.e., decreasing the recording bit size, maintaining the thermal stability of recording magnetization, and maintaining (reducing) the recording magnetic field. That is, a so-called “trilemma” occurs.
As a means for solving this “trilemma”, a novel magnetic recording medium called a percolated medium has recently been proposed. Unlike the granular film type recording layer, a recording layer of the percolated medium has a continuous-film-like grain structure in which magnetic crystal grains are not surrounded by a nonmagnetic grain boundary region, and a strong exchange interaction acts between the magnetic crystal grains. Pinning sites for pinning magnetic domain walls are formed in the recording layer of the percolated medium by some method, thereby suppressing the spread of the domain walls. This forms a fine magnetic domain structure corresponding to the density of the pinning sites. The recording bit size is decreased by forming these pinning sites at a density higher than that of the magnetic crystal grains in the existing granular film type recording layer. In the percolated medium recording layer, the recording bit size is independent of the magnetic crystal grain size. Unlike the granular film type recording layer, therefore, the magnetic crystal grain size need not be decreased, and this makes it possible to maintain the thermal stability and decrease the recording bit size at the same time. Also, since the magnetization reversing mechanism differs from that of the granular film type recording layer, the recording magnetic field hardly increases even when the Ku of the magnetic crystal grains is increased. As described above, the percolated medium is an epoch-making magnetic recording medium that can overcome the “trilemma” when increasing the recording density of the HDD medium.
Since, however, no practical means for forming fine pinning sites at a high density in a continuous magnetic film has been developed at present, no percolated medium has been put into practical use.